When the moisture content of an electrolyte membrane in a polymer electrolyte membrane fuel cell decreases, the proton conductivity of the electrolyte membrane also decreases, and as it does so, the membrane resistance increases. As a result, the output voltage decreases, thus reducing battery performance. In order to minimize this kind of problem, Japanese Patent Application Publication No. 2002-175821 (JP-A-2002-175821), for example, proposes a structure that performs control to further increase the gas pressure on the cathode side as a countermeasure for when the moisture content of the electrolyte membrane is low.
In a fuel cell, water is produced at the cathode as a result of an electrochemical reaction so almost all of the moisture content discharged from the fuel cell to the outside is discharged as water vapor together with cathode off-gas. If the gas pressure on the cathode side is increased as described above, the water tends to be present in the cathode side flow path in the form of liquid water rather than water vapor, so the amount that is discharged as water vapor together with the cathode off-gas is able to be suppressed. Further, when increasing the gas pressure on the cathode side, a back pressure valve for adjusting the gas pressure on the cathode side is provided at the outlet portion of the gas flow path on the cathode side. When this back pressure valve is controlled in the close direction such that the opening amount of the valve is reduced, the amount of water vapor discharged from the fuel cell is physically suppressed, which reduces the chances of the electrolyte membrane drying out. Moreover, increasing the gas pressure on the cathode side relative to the anode side promotes the movement of water in the electrolyte membrane from the cathode side, where the water is produced, toward the anode side, thereby ultimately increasing the moisture content in the electrolyte membrane.
However, the gas is usually supplied to the cathode side by supplying pressurized air using a pump or the like. Therefore, increasing the gas pressure on the cathode side results in greater power consumption by the pump or the like, i.e., results in greater loss from the auxiliary device. Hence, when the loss from the auxiliary device increases in this way, the energy efficiency of the entire system that includes the fuel cell decreases.